1. Field of the Invention
The present invention relates to a vehicle-behavior control apparatus and method for stabilizing a turning behavior of a vehicle and, more particularly, to a vehicle-behavior control apparatus and method for controlling a turning behavior of a vehicle provided with a differential gear mechanism.
2. Related Background Art
There are conventionally known vehicle-behavior control systems for controlling the turning behavior of the vehicle. For example, Japanese Patent Application Laid-Open No. 8-310366 discloses such technology that, under conditions in which the vehicle is transferring into a spin tendency or into a drift-out tendency, braking force at each wheel is adjusted according to each of behavioral states so as to restrain this transfer.
For example, in the case of vehicles equipped with a center differential for transmitting the driving force of the engine to a front-wheel drive shaft and a rear-wheel drive shaft with permitting a difference in rotation between them, once the differential mechanism of the center differential is locked by driver""s manipulation, wheel rotation speeds of the front and rear wheels will suffer restriction from each other, and in this state the sum of wheel rotation speeds of the front left and right wheels will always agree with the sum of wheel rotation speeds of the rear left and right wheels. Supposing the braking force is exerted on the front outside wheel to decrease the wheel speed thereof to some extent, either of the following effects will appear by the degree of the decrease in the rotation speed of the front outside wheel: (a) the front inside wheel increases its speed; (b) the rear inside wheel decreases its speed; or (c) both of the rear wheels decrease their speed. In the case of the front-wheel-drive vehicles or the front-wheel-drive-based four-wheel-drive vehicles, processing for estimating the vehicle speed is carried out based on the wheel speed of the front inside wheel. Therefore, the estimated vehicle speed becomes higher in the case of (a). This causes a target speed of the front outside wheel to be set higher, thereby resulting in weakening the braking force at this wheel. If the rear inside wheel is retarded to undergo braking as in (b), a moment to promote the spin tendency will act to the body. If braking is also effected further at the rear outside wheel as in (c), lateral forces on the rear wheels will decrease, so as to affect stability of vehicle behavior.
Therefore, if the same turning behavior control is carried out when applying the braking force to the outside wheels regardless of locking or unlocking of the center differential, the effect of the turning behavior control of vehicle will not be exhibited fully in certain cases, because an antispin moment, a front-rear balance between wheels lateral forces, etc. depending on locking or unlocking of the center differential.
The above illustrated description is concerning about the center differential as a differential gear mechanism, the same goes for a front differential disposed on the front axle or a rear differential disposed on the rear axle.
The present invention has been accomplished in order to solve the above problem and an object of the invention is to provide a vehicle-behavior control apparatus capable of carrying out suitable turning behavior control according to lock status of the differential mechanism.
A vehicle-behavior control apparatus according to the present invention comprises of braking mechanisms provided for the respective wheels and applying a braking force to a corresponding wheel, and a braking force control means for controlling the braking force applied by each of the braking mechanisms in accordance with a behavior state of the vehicle. Wherein the braking force control means changes the braking force applied by each of the braking mechanisms, depending on whether or not the differential mechanism is in a differential limited state to block differential action thereof.
The vehicle-behavior control apparatus according to the present invention may have a control unit adopted to be connected to a braking system and vehicle status sensors. This control unit directs the braking system to distribute suitable braking force to each wheels in response to a spin or driftout moment determined by any outputs of the vehicle sensors and a state of the center differential determined by a differential state sensor.
A vehicle-behavior control method according to the present invention comprises of (1) estimating a spin and driftout moment and a lock state of the center differential and (2) controlling braking force distributions in response to this estimating result.
Since the control unit (braking force control means) changes the braking forces applied to the wheels by the braking mechanisms in consideration of the differential limited state of the center differential (differential mechanism), the suitable turning behavior control can be carried out, taking the differential limited state of the differential mechanism into consideration.
The differential mechanism may be a center differential for distributing the driving force to the front wheels and to the rear wheels. In the differential limited state of the center differential, the wheel rotation speeds of the front and rear wheels suffer restriction from each other and the braking force and/or the driving force acts to unexpected wheels. This can result in change in the magnitude of the antispin moment generated and in the front-rear balance between tire lateral forces, when compared with those in the free state of the center differential. Preferably the braking force control means thus sets the braking force generated at each braking mechanisms in consideration of such conditions.
The braking force control means may apply an excess braking force to the front outside wheel, whereby the antispin moment is exerted on the vehicle to restrain the spin tendency of vehicle behavior, thereby stabilizing the behavior of the vehicle.
If the braking force control of each braking means is carried out in order to stabilize the turning behavior of the vehicle in the differential limited state of the differential mechanism, there will also arise such cases that the effect of the behavior control is not demonstrated fully. Therefore, the braking force control by the braking force control means is preferably disabled when the differential mechanism is in the differential limited state.
In the differential limited state of the differential mechanism, the braking force at the front wheels (particularly, at the front outside wheel) acts as circulating torque to the rear wheels. When the road friction force is large, the vertical load decreases greatly, particularly, at the rear inside wheel because of a load shift due to the centrifugal force. This raises the possibility of increasing slip amounts. For this reason, the braking force control by the braking force control means is preferably disabled when the differential mechanism is in the differential limited state and when the road friction force is large.
Since the stability of the vehicle becomes lower with increase in the vehicle speed, the apparatus may be constructed so that the braking force control by the braking force control means is carried out with priority during running at high speed even in the differential limited state of the differential mechanism in order to stabilize the vehicle behavior, whereas the braking force control by the braking force control means is disabled in the low speed range of vehicle speed.
When the traction is suppressed, the vehicle speed is decreased, which acts advantageously in terms of running stability. For this reason, it is preferable to suppress the traction by traction suppressing means even if the braking force control by the braking force control means is disabled under either of the aforementioned conditions in the differential limited state of the differential mechanism.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.